In many applications wherein electrochemical or photoelectrochemical cells are used, such as in solar cells or data displays, there is a need to expose an electrolyte to a very large electrode area. This could be expressed such that the electrode should exhibit a very high surface to volume ratio.
A useful method to achieve a high surface to volume ratio electrode is to manufacture an electrode in the form of a nanostructured film, i.e. a network of interconnected particles of nanometer size. The porosity of such a film is typically in the range of 50-60%. The particles are typically of a semiconductor material, such as a metal oxide, and the particle size is typically within the range of from a few nanometers up to several hundred nanometers. The thickness of a nanostructured film is typically in the order of 5-10 μm, but may be up to several hundred μm.
The electrode film is deposited on a substrate, such as a glass sheet. However, the nanostructured film must be electrically connected to peripheral devices. Since the substrate typically is an insulator, a conducting layer is provided on the substrate and the nanostructured electrode is deposited on the conducting layer. A substrate (e.g. glass) coated with a conducting layer is called a conducting substrate (such as a conducting glass).
The function of the nanostructured film depends on the application. For example, in a solar cell the function of the nanostructured film is to collect electrons from an excited state produced when light is being absorbed in dye molecules attached to the surface of the nanostructured film. The electrons are transported through the particle network of the film to the conducting substrate where the photocurrent is collected. In display applications, on the other hand, the nanostructured film is useful to deliver electrons to surface attached dye molecules or to the nanostructured surface in itself to accomplish intercalation of, for instance, lithium ions. By changing the electrical potential of the conducting substrate the apparent color of the nanostructured film is controlled.
There are several previously known methods for manufacturing nanostructured films. Common to most of them is that the semiconductor material is applied to the conducting substrate in the form of very small particles, typically with a size of a few nanometers, present in a colloidal solution. These small particles are physically and electrically connected using a firing process. The firing process is performed at a temperature of several hundred degrees and for a time period of, typically, half an hour.
Actually, in addition to the firing process described above, conventional methods for forming nanostructured films include several steps, each step often rather time and cost consuming. For example, a colloidal solution preparation step includes measures to ensure a low degree of particle aggregation, such as adding organic additives. Thus, the firing process is needed not only to connect the particles, but also to remove the anti-aggregating organic additives in the colloidal solution by combustion. Furthermore, a film deposition step may include the use of screens to pattern or limit the extension of the film.
The firing step of conventional methods for forming nanostructured films also sets limits to the choice of substrate material. The high firing temperature, especially in combination with a long dwell time, rules out plastics as substrate materials.
The method of U.S. Pat. No. 4,054,478 is an example of a method that requires an intermediate step wherein particles are temporarily connected using a binder in order to provide a structural stability to a film of particles. The binder then has to be removed using a high temperature (such as 350-400° C.) treatment during a step of compression.
The method of U.S. Pat. No. 5,616,366 concerns the manufacturing an electrode current collector assembly wherein a solid electrolyte is used, i.e. the electrodes are not of the nanostructured technique to which the present application is aimed. However, the method of U.S. Pat. No. 5,616,366 is an example of a presently known technique requiring a curing step, in addition to a compressing step, for forming an electrode film.
Therefore, there is a need for a fast nanostructured film manufacturing method that does not require a heat treatment step and thereby allows for the use of a wide range of substrate materials.